Transcutaneous PCO 2 monitoring in critically ill patients: update and perspectives

The physiology of venous and tissue CO 2 monitoring has a long and well-established physiological background, leading to the technological development of different tissue capnometric devices, such as transcutaneous capnometry monitoring (TCM). To outline briefly, measuring transcutaneous PCO 2 (tcPCO 2 ) depends on at least three main phenomena: (I) the production of CO 2 by tissues (VCO 2 ), (II) the removal of CO 2 from the tissues by perfusion (wash-out phenomenon), and (III) the reference value of CO 2 at tissue inlet represented by arterial CO 2 content (approximated by arterial PCO 2 , or artPCO 2 ). For this reason, there are, at present, roughly two clinical uses for tcPCO 2 measurement: a respiratory approach where tcPCO 2 is likely to estimate and non-invasively track artPCO 2 ; and a hemodynamic under-estimate use where tcPCO 2 can reflect tissue perfusion, summarized by a so-called “tc-art PCO 2 gap”. Recent research shows that these two uses are not incompatible and could be combined. The spectrum of indications and validation studies in ICUs is summarized in this review to give a survey of the potential applications of TCM in critically ill patients, focusing mainly on its potential (micro)circulatory monitoring contribution. We strongly believe that the greatest benefit of measuring tcPCO 2 is not to only to estimate artPCO 2 , but also to quantify the gap between these two values, which can then help clinicians continuously and noninvasively assess both respiratory and hemodynamic failures in critically ill patients.